1. Technical Field
This invention relates to systems and methods for determining the position of a mobile unit using ultrasound.
2. Background Information
There are many situations in which it is desirable to be able to determine the location of a moveable object, such as a person or an item of equipment, within an area such as a hospital ward.
It is known to provide a network of static base stations, e.g. attached to ceilings or walls, and to attach ultrasonic tags to moveable objects which communicate with one or more of the base stations so as to allow the location of the tagged object to be determined. Ultrasound is well suited to this purpose as it is undetectable to humans. It also travels much more slowly than radio waves, and attenuates more rapidly, so is much easier to process for determining the typical relatively short distances between transmitter and receiver.
US 2005/0232081 describes a system for indoor acoustic positioning in which an identification tag transmits a spread spectrum sequence unique to that tag. A detector unit receives the sequence and compares it against all the sequences in use in the system in order to identify the tag. The position of the tag can be determined based on differences in arrival times of the sequence at different detector units.
Such an approach has limitations. In particular, its accuracy in deter mining the identity and position of the tag can be adversely affected by multi-path interference, which is commonly present due to reflections of the transmitted signal from walls, ceilings and other surfaces. It can also fail to recognize a tag which is moving and thereby creating a Doppler shift in the received signals. It is also limited to having a fairly small number of tags, since an increase in the number of tags in the system as a whole requires an increase in the lengths of the sequences to ensure that sufficient are available, which can increase computational requirements. Moreover, in order to maintain reasonable update rates, multiple tags must transmit simultaneously, which is likely to lead to recognition errors.
The present invention seeks to address these shortcomings.